During the past year we recorded intracellularly from optic-tract fibers entering the cat's lateral geniculate nucleus, characterized them physiologically and then injected them with horseradish peroxidase. Afterwards we histologically reconstructed the arborizations and terminations of these axons. Our results have been based primarily on a sample of one physiological class of retinal axons, the non-linear Y type. This year we will examine single axons of their functional classes, particularly those from X cells which have linear spatial summation. It would be most important to compare the thalamic and/or midbrain projections of X retinal cells with those of the Y units. For instance, we would like to know if the X optic-tract fibers project to the lateral geniculate nucleus, to the superior collicus and to the medial interlaminar nucleus, as the Y units do. We would like to determine the specific distributions of the X terminals within the various geniculate layers, in the superior colliculus and in the medial interlaminar nucleus. Finally, utilizing these same techniques we will begin a preliminary study of the projection patterns of color-sensitive optic-tract fibers into the geniculate of the monkey.